Orthogonal signal generation system

ABSTRACT

A local signal of preset frequency is supplied to an input terminal of a signal input section. The local signal is supplied to a control input terminal of a variable current source, a current corresponding to the local signal is output from the variable current source, and the current is supplied to a phase shifting section. The phase shifting section includes an integrator constructed by a capacitor and a resistor connected in series and a differentiator constructed by a resistor and a capacitor connected in parallel. As parallely connecting the phase shifting circuit with a linear element of which the signal input section consists, the phase shifting section outputs first and second output signals which has the same frequency as the local signal and whose phases are different from each other by 90°. The output signals of the phase shifting section are output to the next stage via an orthogonal signal output section constructed by current source and transistors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a signal generation system applied to a mobilecommunication device such as a portable wireless telephone or the like,and more particularly to an orthogonal signal generation system forgenerating carrier signals or RF (radio frequency) signals whose phasesare different from each other by 90° and which are used for orthogonalmodulation/demodulation.

2. Description of the Related Art

In recent years, as represented by portable wireless telephones andcellular (radio) phones, mobile communication devices which cancommunicate in desired places are actively developed. For example, sincethis type of communication device is carried by a man or mounted in acar, it is desired to reduce the size and weight of the communicationdevice. For this purpose, parts of the communication device are requiredto be formed in a monolithic IC form suitable for reduction in size andweight rather than in the form of conventional hybrid IC (integratedcircuit). Further, since it is necessary to drive the portable wirelesstelephone by use of a battery, it is desired to develop an IC which isoperated on a low voltage.

In the above-described mobile communication device, an orthogonalmodulation/demodulation system for superposing a speech signal on twocarriers whose phases are different from each other by 90° andtransmitting the speech signal is generally used as a communicationsystem. In order to realize the orthogonal modulation system, it isnecessary to create carrier signals whose phases are different from eachother by 90° from a local signal generated from a local oscillator athigh s/n (signal to noise) ratio. In this invention, a system forgenerating two output signals such as the two carrier signals set in anorthogonal relation is called an orthogonal signal generation system.

In the circuit of the conventional orthogonal signal generation system,a resister for terminating to prevent reflection of the local signal isarranged at an input terminal inputting the local signal. Since thevoltage amplitude of the output signal of the orthogonal signalgeneration circuit is determined by the power of the input signal,causing a problem that a sufficiently high voltage gain cannot beobtained when the resister for terminating is set to 50Ω or 70Ω forexample. In principle, it is possible to increase the voltage gain byselectively setting the value of the terminal resistor, but in the GHzband, one of the values 50Ω and 75Ω can be generally selected because ofthe impedance of the transmission line.

In general, transistors are employed in the circuit of the orthogonalsignal generation system. Assuming that two cascated emitter followerare employed in the circuit and input signals are transmitted via thetransistors, potentials of the output signals from the circuit are setto a low value by a voltage drop between a base and an emitter. Further,Judging from the recent trend of the requirement for operating varioustypes of devices on low voltage, it is anticipated that the power supplyvoltage for portable wireless telephones and the like will be less thanor equal to approx. 2.5 [V], thereby the above voltage drop will becomea serious problem. For example, when the output signals of the circuitare amplified by use of a differential amplifier, the voltage dropoccurs at the common emitter terminal of a pair of emitter-coupledtransistors constructing the differential amplifier. When the powersupply voltage is 2.5 [V] and if V_(BE) of the transistor is approx. 0.7[V], the potential of the common emitter terminal is set to approx. 0.1to 0.4 [V] and it becomes difficult to operate a current sourceconnected to the common emitter terminal. That is, in general mobilecommunication devices, the circuit following the orthogonal signalgeneration system will be disable for operating by the voltage drop.

In order to solve the above problem, it is possible to insert acapacitor between the output terminals of the circuit and thedifferential amplifier to cut off the DC component and shift the DClevel of the input of differential amplifier by use of a level shiftingcircuit. However, with this construction, since it is necessary to formthe level shifting circuit including the DC cut-off capacitor on an IC,the chip area and the lost of the power of the local signals areincreased and the cost is raised and it is not preferable.

As described before, in the conventional orthogonal signal generationsystem, since the voltage (potential) of the output terminal issignificantly lowered in comparison with a power supply voltage suppliedto the system, it is not suitable for low voltage operation. Further, inthe conventional orthogonal signal generation system, since it isrequired to attach a terminal resistor to the input side of a localsignal, a sufficiently high voltage gain cannot be obtained.

SUMMARY OF THE INVENTION

A first object of this invention is to provide an orthogonal signalgeneration system capable of creating two output signals whose phasesare different from each other by 90° with high precision and providing ahigh voltage gain.

A second object of this invention is to provide an orthogonal signalgeneration system which can create two output signals whose phases aredifferent from each other by 90° with high precision and which can beoperated on low voltage.

According to a first aspect of this invention, there is provided anorthogonal signal generation system which comprises current controlmeans including a variable current source, for controlling a currentvalue according to an input signal; phase shifting means for outputtingfirst and second signals whose phases are different from each other by90°; and a power supply for supplying a preset voltage to the currentcontrol means and phase shifting means.

In the orthogonal signal generation system, the current control meansincludes a linear element connected between the power supply and thevariable current source, and the phase shifting means includes anintegrator and a differentiator which are connected in parallel with thelinear element.

According to a second aspect of this invention, there is provided anorthogonal signal generation system which comprises first currentcontrol means including a first variable current source, for controllinga first current value according to an input signal; second currentcontrol means including a second variable current source, forcontrolling a second current value according to an inverted signal ofthe input signal; first phase shifting means for outputting a firstsignal and a second signal which is shifted from the first signal by aphase of 90°; second phase shifting means for outputting a third signaland a fourth signal which is shifted from the third signal by a phase of90°; output means for outputting a first differential signal between thefirst and third signals and a second differential signal between thesecond and fourth signals; and a power supply for supplying a presetvoltage to the first and second current control means and the first andsecond phase shifting means.

In the orthogonal signal generation system with the above construction,a current of an amplitude corresponding to an AC signal is created inthe variable current source, and the current is supplied to the linearelement and the integrator and differentiator of the phase shiftingcircuit. As a result, output signals whose phases are different fromeach other by 90° are output from the integrator and differentiator.

By adjusting a bias current of the variable current source, the inputimpedance can be set to a specified value without attaching a terminalresistor to the input side. As a result, the voltage gain can beincreased.

Further, the potential of an input node of the phase-shifter, that is,the potential of the terminal connected to the variable current sourcevia the linear element is set to the potential level subtracted avoltage drop of the linear element from the potential level of the powersupply, and unlike the conventional case, it is not lowered by thebase-emitter voltage of the transistor. Therefore, the operating voltagerange is enlarged and the low-voltage operation can be effected. Inparticular, as the above voltage drop does not occur when the linearelement consists of an inductance element, it is possible to operate theorthogonal signal generation system by further low-voltage of the powersupply. Further, it is not necessary to attach a DC cut-off capacitor tothe output side, thereby making it possible to attain the constructionwhich is suitable for formation of the monolithic IC and to get highgain because there is no power loss by a parasitic of DC cut offcapacitor.

Further, since the DC potentials of the outputs of the integrator anddifferentiator of the phase shifting circuit can be both set to the samepotential level as the power supply potential, parasitic capacitorsassociated with transistors constructing buffers can be made equal toeach other when the buffers such as emitter followers are provided onthe output sides of the integrator and differentiator. As a result, thephase error and amplitude error of the output signal caused by adeviation in the parasitic capacitors can be suppressed.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a circuit diagram showing the construction of the orthogonalsignal generation system according to a first embodiment of the presentinvention;

FIG. 2 is an equivalent circuit diagram of a phase shifting circuitshown in FIG. 1;

FIG. 3 is a block diagram showing the construction of an orthogonalsignal generation system according to a second embodiment of thisinvention;

FIG. 4 is a diagram showing the circuit construction of the orthogonalsignal generation system shown in FIG. 3;

FIG. 5 shows a first example of an equivalent circuit of a phaseshifting section shown in FIG. 4;

FIG. 6 shows a second example of the equivalent circuit of the phaseshifting section shown in FIG. 4;

FIG. 7 is an equivalent circuit diagram of a variable current sourceshown in FIG. 4;

FIG. 8 is a circuit diagram showing a first concrete example of thevariable current source shown in FIG. 4;

FIG. 9 is a circuit diagram showing a second concrete example of thevariable current source shown in FIG. 4;

FIG. 10 is a block diagram showing the construction of an orthogonalsignal generation system according to a third embodiment of thisinvention;

FIGS. 11A to 11C are showing a modulator and a demodulator to which theorthogonal signal generation system of this invention is applied;

FIG. 12 is a block diagram showing the construction of a receivingsection of a communication device to which a super-heterodyne system isapplied;

FIG. 13 is a block diagram showing the construction of a demodulatorshown in FIG. 11 to which the orthogonal signal generation system ofthis invention is applied; and

FIG. 14 is a block diagram showing the construction of a communicationdevice to which a direct conversion system in which the orthogonalsignal generation system of this invention is used is applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

At first, a circuit shown in FIG. 1 according to the first embodiment ofthe present invention is provided in order to realize the orthogonalsignal generation system. This circuit of the orthogonal signalgeneration system of the first embodiment is explained below withreference to FIGS. 1 and 2.

FIG. 1 shows the first provided circuit construction of the orthogonalsignal generation system. A local signal input to an input terminal 100is terminated at a terminal resistor 102 via a capacitor 101 and theninput to a phase shifting circuit 105 via an emitter follower circuitwhich is constructed by a transistor 103 and a current source 104. Thephase shifting circuit 105 includes a differentiator constructed by acapacitor C11 and a resistor R11 and an integrator constructed by aresistor R12 and a capacitor C12. A node N10 between the resistor R11and the capacitor C12 is connected to a low impedance such as a powersupply VCC or ground terminal, for example. The phase shifting circuit105 outputs a signal whose phase is advanced with respect to the phaseof a local signal from the differentiator and a signal whose phase isdelayed with respect to the phase of a local signal from the integrator.An output signal of the differentiator is supplied to an output terminal110 via an emitter follower circuit constructed by a transistor 106 anda current source 108, and an output signal of the integrator is suppliedto an output terminal 111 via an emitter follower circuit constructed bya transistor 107 and a current source 109. Since the circuit is assumedto deal with a signal of GHz band, the circuit is constructed to have aterminal resistor 102 for impedance matching and permit only an ACcomponent of the local signal to be supplied to the phase shiftingcircuit 105 by use of the capacitor 101.

In the orthogonal signal generation system, two output signals whosephases are different from each other by 90° and whose amplitudes areequal to each other are supplied to the output terminals 110, 111 bydetermining the element value as follows.

    R11=R12, C11=C12

    1/(ωc*C11)=R11                                       (1)

where ωc is a local signal frequency.

FIG. 2 shows an equivalent circuit of the phase shifting circuit shownin FIG. 1. R20 denotes an equivalent resistor of the transistor 103 asviewed from the emitter side thereof, and one end of the resistor isconnected to a signal source Vin and the other end thereof is connectedto the phase shifting circuit 105. C21, C22 denote parasitic capacitorssuch as a capacitor between the capacitor electrode and the ground. C11,C12 denote base-collector capacitors (Cμ) of the transistors 106, 107.

If the parasitic capacitors C21 and C22 are equal to each other, twooutput signals having no phase difference and no amplitude differencecan be derived from the output terminals 110 and 111. However, if theparasitic capacitors C21 and C22 are made different from each other by adifference in the base-collector voltage of the transistors 106 and 107or a variation in the manufacturing process, the phase relation andamplitude ratio between the two output signals derived from the outputterminals 110 and 111 are changed. Therefore, in general, the sizes ofthe transistors 106, 107 are reduced so that the ratio of the capacitorsC11, C12 to the parasitic capacitors C21, C22, i.e. the ratios of thecapacitor C21 to the capacitor C11 and the capacitor C22 to thecapacitor C12, can be reduced to suppress the influence by a fluctuationin the parasitic capacitors C21, C22.

In the circuit of the orthogonal signal generation system shown in FIG.1, a sufficiently high voltage gain can not be attained. Because inputvoltage is smaller as inputimpedance is smaller when applied power isconstant. That is, input voltage decreases if inputimpedance is smallsuch as 50Ω or 75Ω. As will be described in detail later, the maximumgain of the output signal to the input signal of the conventionalorthogonal signal generation system is 1/√2. In principle, it ispossible to increase the voltage gain by selectively setting the valueof the terminal resistor 102, but in the GHz band, only one of thevalues 50Ω and 75Ω can be generally selected because of the impedance ofthe transmission line.

Further, the circuit constructing the orthogonal signal generationsystem has an advantage that the high precision can be maintained in arange up to the GHz band, but it has a disadvantage that it is notsuitable for low-voltage operation by the following reason.

In FIG. 1, the potentials of the output terminals 110, 111 are set atmaximum to a value which is lower than the power supply voltage +VCC by2V_(BE) (V_(BE) is a base-emitter voltage of the transistor). Forexample, the potential obtained at the output terminal 110 is set to apotential (+VCC-V_(BE)) which is lower than +VCC by V_(BE) of thetransistor 106, and the potential obtained at the output terminal 111 isset to a potential (+VCC-2V_(BE)) which is lower than +VCC by V_(BE) ofthe transistors 103, 107.

Assuming that the output signals from the output terminals 110, 111 aregenerally amplified by use of a differential amplifier, the potential ofthe common emitter terminal of a pair of emitter-coupled transistorsconstructing the differential amplifier is further lowered by V_(BE) ofthe transistor, and as a result, a voltage drop of 3V_(BE) with respectto the power supply voltage +VCC will occur.

Next, there will now be described second and third embodiments of thisinvention with reference to the accompanying drawings.

FIG. 3 shows the schematic construction of an orthogonal signalgeneration system according to a second embodiment of this invention.The orthogonal signal generation system 10 includes a signal inputsection 1, phase shifting section 2, and orthogonal signal generationoutput section 3. Each section is supplied with a power supply voltage+VCC.

The signal input section 1 generates a current corresponding to thesignal generated from a local oscillator, and supplies the current tothe phase shifting section 2. The phase shifting section 2 has anintegrator and a differentiator and supplies signals (orthogonalsignals) whose phases are different from each other by 90° to theorthogonal signal output section 3 according to the current suppliedfrom the signal input section 1. The orthogonal signal output section 3has a buffer circuit and transmits the orthogonal signals supplied fromthe phase shifting section 2 to the next stage via output terminals 31,32.

An example of the concrete circuit construction of the above orthogonalsignal generation system is shown in FIG. 4. An AC input signal, forexample, local signal VLO generated from a local oscillator (not shown)is supplied to the input terminal 11 of the signal input section 1 shownby chain lines in FIG. 4. The local signal VLO is supplied to a controlinput terminal of a variable current source 12. As a result, a currentwhose value varies according to the local signal VLO is output from thevariable current source 12. One end of the variable current source 12 isconnected to the ground and the other end (node N0) thereof is connectedto a positive power supply +VCC via a linear element 13. The linearelement 13 is constructed by a passive element such as an inductor orresistor, for example. A circuit of the phase shifting section 2 isconnected in parallel with the linear element 13.

The phase shifting section 2 includes an integrator which is constructedby a series circuit of a first capacitor C1 and a first resistor R1 anda differentiator which is constructed by a series circuit of a secondresistor R2 and a second capacitor C2. With the above circuitconstruction, first and second output signals whose phases are differentfrom each other by 90°.

The output node N1 (which is a connection node between the capacitor C1and the resistor R1) of the integrator of the phase shifting section 2and the output node N2 (which is a connection node between the resistorR2 and the capacitor C2) of the differentiator are respectivelyconnected to the bases of transistors 33 and 34 in the orthogonal signaloutput section 3. The orthogonal signal output section 3 has currentsources 35, 36 in addition to the transistors 33, 34. The transistors33, 34 constitute emitter follower circuits in cooperation with therespective current sources 35, 36 and are used as buffer circuits fortransmitting the output signals of the phase shifting section 2 to thenext stage via the output terminals 31, 32. That is, the collectors ofthe transistors 33, 34 are connected to the power supply +VCC and theemitters thereof are respectively connected to one-side ends of thecurrent sources 35, 36 and to the output terminals 31, 32. The otherends of the current sources 35, 36 are connected to the ground GND.

Next, the operation of the orthogonal signal generation systemconstructed as shown in FIG. 4 is explained with reference to theequivalent circuit of FIG. 5. Assume that the linear element 13 is aninductor of inductance L and C1=C2 and R1=R2 as shown in FIG. 5. An ACcomponent ILO (which is hereinafter referred to as a local signalcurrent) which is contained in the current output from the variablecurrent source 12 and which corresponds to the local signal VLO isdistributed to the linear element 13, integrator (C1, R1) anddifferentiator (C2, R2). In this case, if the current flowing in theintegrator is Iint and the current flowing in the differentiator isIdif, then the currents Iint and Idif can be expressed by the followingequation. ##EQU1## where C=C1=C2, R=R1=R2, and ωc is a local signalfrequency.

It is understood from the equation (2) that the local signal current ILOcan be input to the phase shifting section 2 with the current gain ofsubstantially 1 by increasing the inductance L. Further, if theresonance frequency of L and C is set equal to the local signalfrequency fc, |Iint|, |Idif| are set less than 0.67 and the ratio(current gain) of the local signal current ILO to the input current(currents flowing in the integrator and differentiator) of the phaseshifting section 4 can be set equal to or larger than 0.5. As a gain ofthe orthogonal signal generation system is related to values of theresistors and capacitors that constitute the phase shifting section 2,the total gain is explained below.

As described before, orthogonal signals or first and second outputsignals whose phases are different from each other by 90° can be derivedfrom the integrator and differentiator by inputting the local signal ILOto the integrator and differentiator of the phase shifting section 2.The first and second output signals are respectively supplied to theoutput terminals 31, 32 via the emitter follower circuits which arerespectively constructed by the transistor 33 and current source 35 andthe transistor 34 and current source 36. A differential amplifier isgenerally connected to the succeeding stage of the orthogonal signaloutput section 3, that is, output terminals 31, 32. The differentialamplifier and the phase shifting section 2 are electrically isolatedfrom each other by the emitter follower circuit.

In the orthogonal signal generation system of the first embodiment, whenthe linear element consists of inductance element, since the potentialof the input node N0 of the phase shifting section 2 is set to the samepotential level as the power supply +VCC, the operation voltage rangecan be enlarged by the base-emitter voltage V_(BE) of the transistor incomparison with the conventional circuit construction shown in FIG. 1.That is, in the conventional circuit shown in FIG. 1, the potentials ofthe output terminals 110, 111 are set to the potential level which islower than +VCC by 2V_(BE), but in the circuit of this embodiment shownin FIG. 4, the potentials of the output terminals 31, 32 are set to thepotential level which is lower than VCC only by V_(BE).

Therefore, assuming that the output signals from the output terminals31, 32 are input to the differential amplifier, the potential of thecommon emitter terminal of the paired emitter coupled transistors in thedifferential amplifier is lowered than +VCC only by 2V_(BE). As aresult, the potential of the common emitter terminal becomes equal to orhigher than 1 [V] and the current source connected to the common emitterterminal can be operated without causing any problem even when +VCC islowered to approx. 2.5 [V] in a portable wireless telephone, forexample.

Further, in this embodiment, since the potentials of the output nodesN1, N2 of the integrator and differentiator of the phase shiftingsection 2 are set equal to +VCC, the base-collector capacitances (Cμ) ofthe transistors 33, 34 becomes equal to each other at the DC operatingpoint. On the other hand, in the conventional case shown in FIG. 1,since it is difficult to set the base potentials of the transistors 106,107 equal to each other, it is difficult to set the capacitances Cμ ofthe transistors equal to each other. A deviation in the capacitance Cμ(parasitic capacitance) caused by the DC bias causes a phase error andamplitude error in the orthogonal signals created by the orthogonalsignal generation system. Therefore, the circuit of this embodiment isimproved over the conventional circuit in the phase and amplitudeprecision.

In the first embodiment, the inductor is used as the linear element 13as shown in FIG. 5, but a resistor R may be used as shown in FIG. 6. Inthis case, the potential of the input node N0 of the phase shiftingsection 2 is slightly lowered from +VCC by a current flowing in theresistor R and the operation voltage range is narrowed in comparisonwith a case where the inductor is used as the linear element 13.However, it is easy to design that a voltage drop across the resistor Rcan be made smaller than the base-emitter voltage V_(BE), for example,it can be set to approx. 0.3 [V]. Therefore, the operation voltage rangecan be enlarged by at least approx. 0.4 [V] in comparison with thecircuit used in the conventional case. Further, the base-collectorcapacitances of the transistors 33, 34 are made different from each bythe voltage drop across the resistor R, but the difference between themis smaller than that in the circuit of the conventional case.

Next, the construction and operation of the variable current source 12are explained with reference to the accompanying drawings. When thefrequency of the local signal VLO which is an AC input signal of theorthogonal signal generation circuit is a frequency of GHz band, it isnecessary to attain the impedance matching with the transmission linefor transmitting the local signal from the local oscillator in thesignal input section 1 of the orthogonal signal generation system 10 inorder to input the local signal from the local oscillator. Thecharacteristic impedance of the transmission line is generally 50Ω, andin this case, the input impedance of the orthogonal signal generationsystem 10 is set to 50Ω.

FIG. 7 shows an equivalent circuit of the variable current source 12shown in FIG. 4 in the signal input section 1 of the orthogonal signalgeneration system 10. The input impedance of the input terminal 11 is50Ω. Assume that the local signal current ILO flowing in the variablecurrent source 12 has the relation expressed by the following equationwith respect to the potential of the input terminal 11, that is, thevoltage of the local signal VLO.

    IL0=gm*VL0                                                 (3)

where gm is a transconductance.

By use of the above variable current source 12, the local signal VLO canbe converted into a local signal current ILO with high linearity.

FIG. 8 shows a first concrete example of the variable current source 12realizing the equivalent circuit shown in FIG. 7. The input terminal 11is connected to the emitter of a transistor 121 and to one end of acurrent source 122. The other end of the current source 122 is grounded.The base of the transistor 121 is grounded via a voltage source VBB. Thecollector of the transistor 121 is a current output terminal of thevariable current source 12 and corresponds to the node N0 shown in FIG.4. The input impedance Rin of the variable current source 12 can beapproximately expressed by the following equation by setting the currentof the current source 122 to Itail.

    Rin=1/gm                                                   (4)

where gm=Itail/Vt, Vt (thermal voltage)=kT/q, k is the Boltzmann'sconstant, and q is the charge of an electron.

The input impedance Rin can be set to 50Ω by adjusting the bias currentof the transistor 121, that is, the current Itail of the current source122. Therefore, the current gain of the variable current source 12 isset to 1 according to the equations (3) and (4) and the output currentILO becomes equal to the AC component of the current based on the localsignal VLO input to the input terminal 1.

FIG. 9 shows a second concrete example of the variable current source 12realizing the equivalent circuit shown in FIG. 7. As shown in FIG. 9,the circuit construction of the second concrete example is obtained byreplacing the current source 122 shown in FIG. 8 by a resistor 123.Since the potential of the input terminal 11 is given by (VBB-0.7 [V])when the transistor 121 is operated, a desired current Itail can beobtained by setting the resistance R123 of the resistor 123 to a valueexpressed by the following equation.

    Itail=(VBB-0.7)/R123                                       (5)

In this case, since the input impedance Rin is a parallel resultantresistance of the impedance 1/gm as viewed from the emitter side of thetransistor 121 and the resistance R123, Rin=50Ω can be realized with acurrent smaller than Itail given in FIG. 8. At this time, the currentgain of the variable current source 12 is lowered by a conductance of1/R123, but it is easy to obtain a current gain equal to or larger than0.8.

Next, the voltage ratio of the local signal VLO input to the inputterminal 11 to the output signal of the orthogonal signal generationsystem 10, that is, the voltage gain is explained in comparison withthat of the circuit in the conventional case shown in FIG. 1. In thiscase, assume that the output impedance of the signal source connected tothe input terminal 11 is 50Ω and transistors and passive elements usedin the signal source and the orthogonal signal generation system are allideal parts.

In the conventional circuit shown in FIG. 1, if the voltage of thesignal source is Vin and the resistance of the terminal resistor 102 is50Ω, then the potential VN20 of the node N20 is expressed by thefollowing equation.

    VN20=Vin/2                                                 (6)

Since the voltage gain of the emitter follower constructed by thetransistor 103 and the current source 104 is ideally set to 1, an inputvoltage of the phase shifting circuit 105 is equal to the voltage of thenode N20. Under the condition of the equation (1), the output signalsV11, V12 of the phase shifting circuit 105 are expressed by thefollowing equation irrespective of the values of the capacitors and theresistors.

    V11=V12=1/(2·√2))Vin                       (7)

Therefore, the gain of the conventional circuit is set to 1/(2×√2) atmaximum in a system which is designed by taking the impedance matchinginto consideration.

On the other hand, in the orthogonal signal generation system of thisinvention, the current gain is set to 1 as described before, and thefollowing equation can be attained.

    IL0=Vin/100                                                (8)

Therefore, the output voltage V2 of the integrator is expressed by thefollowing equation.

    |V2|=|1/(jωcL-1/ωc.sup.2 CL+2)|·R2·Vin/100              (9)

A value of approx. 0.67 can be obtained as the term of the absolutevalue in the equation (9) by adequately setting the local signalfrequency to the resonance frequency of C and L, and the followingexpression can be obtained.

    |V2|≦0.0067*R2*Vin                (10)

In order to make the voltage gain of the orthogonal signal generationsystem higher than the voltage gain of the conventional case, thecondition expressed by the following expression may be satisfied.

    0.067*R>1/(2·√2)                           (11)

Based on the above expression, the following expression can be obtained.

    R>52Ω                                                (12)

Therefore, by setting the resistance R2 equal to or larger than 52Ω, thevoltage gain can be made higher than that of the circuit of theconventional orthogonal signal generation system shown in FIG. 1.

If actual numeric values are used, for example, if the local signalfrequency fc is set to 2 GHz, the element values of the respectivecircuit sections are set such that C=C1=C2=0.6 pF, R=R1=R2=140Ω, andL=10 nH, then the current gain can be set to 0.93 according to theequation (9) and is approximately twice as high as that of theconventional case. In order to obtain the gain equivalent to that of theconventional case, the numeric values may be set under the condition ofthe equation (12) such that C=C1=C2=1.5 pF, R=R1=R2=52Ω, and L=4.2 nH.Therefore, if it is necessary to increase the capacitances of thecapacitors C1, C2 in order to reduce the phase error due to theparasitic capacitor, the gain can be made higher than that of theconventional case when the capacitances are set equal to or less than1.5 pF.

In the first embodiment, the inductor is used as the linear element 13,but it is clearly understood that the gain can be made higher than thatof the conventional case even when a resistor is used as describedbefore. However, since the resonance by L and C cannot be used, the gainis reduced in comparison with a case wherein the inductor is used.

Further, in the embodiments described in this detailed description, thebipolar transistors are used, but GaAs FETs or MOSFETs can be used.

The orthogonal signal generation system can be formed of thedifferential type based on the first embodiment shown in FIG. 4. Theorthogonal signal generation system of the differential type is shown inFIG. 10. As shown in FIG. 10, in the third embodiment, two signal inputsections and two phase shifting sections which respectively correspondto the signal input section 1 and the phase shifting section 2 of thesecond embodiment are used and two differential amplifiers are usedinstead of the orthogonal signal output section 3. That is, theorthogonal signal generation system 20 includes the signal inputsections 1a, 1b, phase shifting sections 2a, 2b, and differentialamplifiers 4a, 4b. Further, the orthogonal signal generation system 20may includes buffer circuits such as the orthogonal signals outputsection 3 shown in FIG. 3 at the following of the phase shiftingsections 2a, 2b respectively.

In the signal input sections 1a, 1b, AC input signals are supplied toinput terminals 11a, 11b in a differential signal form. That is, the ACinput signal supplied to the input terminal 11a and the AC input signalsupplied to the input terminal 11b have phases different from each otherby 180°. The AC input signals supplied to the input terminals 11a, 11bare supplied to the control input terminals of variable current sources12a, 12b.

The signal input section 1a and the phase shifting section 2a constitutea first orthogonal signal generating section, and the signal inputsection 1b and the phase shifting section 2b constitute a secondorthogonal signal generating section. A difference voltage betweenoutput signals from the integrators of the phase shifting sections 2a,2b is amplified by the first differential amplifier 4a. A differencevoltage between output signals from the differentiators of the phaseshifting sections 2a, 2b is amplified by the second differentialamplifier 4b. As a result, first and second output signals whose phasesare different from each other by 90° can be derived from thedifferential amplifiers 4a, 4b. Further, according to the secondembodiment, the design of the differential amplifiers 4a, 4b can besimplified and the high-frequency characteristic can be improved.

As described above in detail, in the orthogonal signal generation systemof this invention, the phase precision and the amplitude precision ofthe output orthogonal signals whose phases are different from each otherby 90° are high and a higher voltage gain can be attained. According tothe orthogonal signal generation system, the low-voltage operation canbe attained, and since the DC cut-off capacitor is not necessary, it canbe suitably formed in the form of monolithic IC. Thus, it is suitablefor a small-sized mobile communication device whose power supply voltageis low.

A modulator 40 and demodulators 50 and 60, each being a directconversion type one, to which this invention is applied, will bedescribed below with reference to FIGS. 11A, 11B and 11C.

As shown in FIG. 11A, the modulator 40 comprises a orthogonal signalgenerator 41 to which the orthogonal signal generation system of theinvention is applied. A local signal Lo is input to the orthogonalsignal generator 41, which generates two signals from the local signalLo. These signals have phase difference 90° between them. The firstsignal is supplied to a multiplier 42a, and the second signal to amultiplier 42b. Two signals Ich and Qch, which are of different bands,are supplied to the multiplier 42a and 42b, respectively. The multiplier42a multiplies the first signal by the signal Ich. The multiplies 42bmultiples the second signal by the signal Qch. The signals output fromthe multipliers 42a and 42b are input to an adder 43. The adder 43 addsthese signals together, generating a signal y(t). The signal y(t) isgiven as follows:

    y(t)=Ich(t)-cos(ωct)-Qch(t)·sin(ωct)  (13)

where cos(ωct) and Ich(t) are the signals supplied to the multiplier42a, and sin(ωct) and Qch(t) are the signals supplied to the multiplier42b.

Since the orthogonal signal generation system can acquire high gain asdescribe above, the proposed orthogonal signal generation system needslower input power compared with conventional orthogonal signalgeneration system. Therefore, using the proposed circuit, local power isreduced. Moreover, because carrier feed-through from input terminal tooutput terminal depends on the input local power, carrier feed-throughis reduced using the proposed circuit.

FIG. 11B shows the demodulator 50 which comprises a orthogonal signalgenerator 51 to which the orthogonal signal generation system of theinvention is applied. A local signal Lo is input to the orthogonalsignal generator 51, which generates two signals from the local signalLo. These signals have phase difference of 90° between them. The firstsignal is supplied to a multiplier 52a, and the second signal to amultiplier 52b. An RF signal is also supplied to the multipliers 52a and52b. The multiplier 52a multiplies the first signal by the RF signal andgenerates a signal Ich. The multiplier 42b multiples the second signalby the RF signal and generates a signal Qch.

Since the orthogonal signal generation system can acquire a large gainas described above, it can generate a signal of the same magnitude(level) as the output signal of the conventional orthogonal signalgeneration system, even if the local signal Lo has a small magnitude.Hence, input power applied to the demodulator 50 is small using theproposed circuit.

FIG. 11C shows the demodulator 60 which comprises a orthogonal signalgenerator 61 to which the orthogonal signal generation system of theinvention is applied. An RF signal is input to the phase-shiftingcircuit 61, which generates two signals from the RF signal. Thesesignals have phase difference of 90° between them. The first signal issupplied to a multiplier 62a, and the second signal to a multiplier 62b.A local signal Lo is also supplied to the multipliers 62a and 62b. Themultiplier 62a multiplies the first signal by the local signal Lo andgenerates a signal Ich. The multiplier 62b multiplies the second signalby the local signal Lo and generates a signal Qch.

Since the orthogonal signal generation system can acquire a large gainas described above, the RF signal input to the orthogonal signalgenerator 61 has a great gain.

Next, an example of the mobile communication device to which thisinvention is applied is explained. FIG. 12 shows the schematicconstruction of a receiver to which a super-heterodyne system isapplied. The receiver 200 includes an antenna 201, high-frequencyamplifier 202, image compressing RF filter 203, mixer 204, localoscillator 205, filter 206, IF amplifier 207, and demodulator 208.

An RF signal received by the antenna 201 is amplified by the RFamplifier 202 and then supplied to the image compressing RF filter 203.The RF signal is subjected to the filtering process in the imagecompressing RF filter 203 so that the image frequency component thereofwill be eliminated. After this, the RF signal is mixed with a carriercreated by the local oscillator 205 in the mixer 204 and the frequencythereof is converted into an intermediate frequency. In general, in thesuper-heterodyne system, such frequency conversion processes areeffected one to three times. As a result, the thus created intermediatefrequency (IF) signal is selected for a desired channel by the filter206 for final channel selection. Further, the intermediate frequencysignal is amplified by the intermediate frequency amplifier (which isnormally an AGC amplifier) 207 and then subjected to the demodulationprocess by the demodulating section 208.

Next, the demodulating section (orthogonal demodulating section) 208 towhich the orthogonal signal generation system of this invention isapplied is explained. In the orthogonal demodulating section 208, theintermediate frequency (IF) signal supplied from the intermediatefrequency amplifier 207 is mixed with a local oscillator signal havingthe same frequency as the former signal, converted into a base band andthen detected.

Now, the operation of the orthogonal demodulating section 208 isexplained with reference to FIG. 13. FIG. 13 shows an example of theconstruction of the orthogonal demodulating section 208. The IF signalsupplied from the intermediate frequency amplifier 207 passes an IFfilter 301 and is then amplified by an IF amplifier 302. Further, the IFsignal is divided into two channels after amplification. The respectivesignals divided into the two channels are mixed with carriers in themixers 303a, 303b. The carriers supplied to the mixers 303a, 303b aresignals whose phases are different from each other by 90° and which arecreated by use of the orthogonal signal generation circuit 305 to whichthe orthogonal signal generation system shown in FIG. 4 is applied, forexample, based on a signal generated from the local oscillator 304.Therefore, the phases of the signals output from the mixers 303a, 303bare different from each other by 90°. By the above process, the IFsignal supplied to the orthogonal demodulating section 208 is convertedinto base band signals.

The thus converted base band signals pass low-pass filters 304a, 304bhaving the anti-aliasing function and are then amplified by base bandamplifiers 305a, 305b. After this, the base band signals are detected bya detector section 306 which effects the process such as delay detectionor synchronous detection, for example. If a digital system is applied tothe detection system, analog/digital converters may be used at thesucceeding stages of the base band amplifiers 305a, 305b.

Further, AC coupling sections 307a, 307b provided at the succeedingstage of the mixers 303a, 303b are used to eliminate the DC component inorder to prevent the amplifiers 305a, 305b from being saturated by DCcomponents generated in the mixers 303a, 303b. Recently, the aboveorthogonal modulating system is applied to various mobile communicationdevices in the demodulation of the orthogonal modulation signal such asa QPSK (quadrature phase shift keying) or QAM (quadrature amplitudemodulation).

Next, a mobile communication device of direct conversion system to whichthis invention is applied is explained with reference to FIG. 14. FIG.14 shows the construction of a communication device 400 having theconstruction for reception/transmission.

During transmitting period, an orthogonal signal generation circuit 401ato which the orthogonal signal generation system of this invention isapplied (in which, for example, the construction shown in FIG. 4 isused) outputs signals whose phases are different from each other by 90°based on a local signal generated from a frequency synthesizer 402. Therespective output signals are multiplied by signals Ich, Qch output froma base band signal generation circuit 403. After this, a differencebetween the multiplied signals is derived and a difference signal issupplied to a variable gain section 404. In the variable gain section404, an input signal is amplified without causing distortion in thesignal and the amplified signal is output to a succeeding-stagepreamplifier 405. A signal output from the variable gain section 404 isoutput to the exterior from an antenna 407 via the preamplifier 405 andband-pass filter 406.

During receiving period, a signal received by the antenna 407 issubjected to the band-pass filtering process by a band-pass filter 408to eliminate signal components of frequencies other than desiredfrequencies and the thus filtered signal is amplified by a linearamplifier 409. After this, the signal is divided into two channels andmultiplied by signals whose phases are different from each other by 90°and which are generated from an orthogonal signal generation circuit401b to which the orthogonal signal generation system is applied. Themultiplied signals are respectively supplied to amplifiers 411a, 411bfor amplification via low-pass filters 410a, 410b. Since thecommunication device shown in FIG. 13 utilizes a digital system,analog/digital converters 412a, 412b are provided at the succeedingstage of the amplifiers 411a, 411b. Output signals of the analog/digitalconverters 412a, 412b are supplied to a base band signal detector 413.

In the above communication device of direct conversion system, thereoccurs a problem that the local signal generated from the frequencysynthesizer 402 leaks into the RF signal. In modulator, this localsignal leaks acceptable for wireless system is specified. Indemodulator, this local signal leaks may cause BER (Bit Error Ratio) toincrease, and may affect other wireless equipment, because this localsignal radiate from antenna via Linear amplifier and Band pass filter.Therefore, when the direct conversion system is used, it is necessary tosuppress the power of the local signal. In the orthogonal signalgeneration system according to this invention, since the high gain canbe attained as described before, the carrier of the same power as thatof the conventional case can be output with the power of the localsignal kept at the low level. Therefore, the problem mentioned above canbe prevented by use of the orthogonal signal generation system.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. An orthogonal signal generation systemcomprising:current control means including a variable current source,for controlling a current value according to an input signal; phaseshifting means for outputting first and second signals whose phases aredifferent from each other by 90° and each of the first signal and thesecond signal having an amplitude corresponding to the controlledcurrent value; and a power supply for supplying a present voltage tosaid current control means and phase shifting means.
 2. An orthogonalsignal generation system according to claim 1, wherein said currentcontrol means includes a linear element connected between said powersupply and said variable current source, and said phase shifting meansincludes an integrator and a differentiator which are connected inparallel with said linear element.
 3. An orthogonal signal generationsystem according to claim 2, wherein said linear element includes aninductor, each of said integrator and differentiator includes acapacitor connected in parallel with said inductor, and a resonancefrequency of the parallel circuit of said inductor and said capacitor ofsaid integrator and differentiator is set equal to a frequency of theinput signal.
 4. An orthogonal signal generation system according toclaim 2, wherein said linear element includes a resistor.
 5. Anorthogonal signal generation system according to claim 2, furthercomprising output means supplied with a preset voltage from said powersupply and including first and second emitter followers each comprisinga transistor and a current source, said first emitter followeroutputting the first signal output from said phase shifting means to anexternal circuit and said second emitter follower outputting the secondsignal from said phase shifting means to the external circuit.
 6. Anorthogonal signal generation system according to claim 2, wherein saiddifferentiator includes a series circuit of a first capacitor and afirst resistor and said integrator includes a series circuit of a secondcapacitor and a second resistor.
 7. A communication device of superheterodyne system comprising:demodulating means for demodulatingsignals, comprising,current control means including a variable currentsource and a liner element connect to said variable current source, forcontrolling a current value according to an input signal, phase shiftingmeans, including an integrator and a differentiator which are connectedin parallel with said liner element, for outputting first and secondsignals whose phases are different from each other by 90° and each ofthe first signal and the second signal having an amplitude correspondingto the controlled current value, and a power supply, connected to saidliner element, for supplying a preset voltage to said current controlmeans and phase shifting means.
 8. A communication device of directconversion system comprising:frequency converting means for converting abase band signal of a transmission portion of said communications deviceinto an RF (radio frequency) signal and for converting an RF signal of areception portion of said communications device into a base band signal,comprising,current control means including a variable current source anda liner element connect to said variable current source, for controllinga current value according to an input signal, phase shifting means,including an integrator and a differentiator which are connected inparallel with said liner element, for outputting first and secondsignals whose phases are different from each other by 90° and each ofthe first signal and the second signal having an amplitude correspondingto the controlled current value, and a power supply, connected to saidliner element, for supplying a preset voltage to said current controlmeans and phase shifting means.
 9. A demodulator comprising:currentcontrol means including a variable current source and a liner elementconnect to said variable current source, for inputting an RF (radiofrequency) signal as an input signal and for controlling a current valueaccording to the input signal; phase shifting means, including anintegrator and a differentiator which are connected in parallel withsaid liner element, for outputting first and second signals whose phasesare different from each other by 90° and each of the first signal andthe second signal having an amplitude corresponding to the controlledcurrent value; a power supply, connected to said liner element, forsupplying a preset voltage to said current control means and phaseshifting means; and multiply means for inputting a local signal, formultiplying the input local signal and the first signal and formultiplying the input local signal and the second signal.
 10. Anorthogonal signal generation system comprising:first current controlmeans including a first variable current source, for controlling a firstcurrent value according to an input signal; second current control meansincluding a second variable current source, for controlling a secondcurrent value according to an inverted signal of the input signal; firstphase shifting means for outputting a first signal and a second signalwhich is shifted from the first signal by a phase of 90° and each of thefirst signal and the second signal having an amplitude corresponding tothe controlled first current value; second phase shifting means foroutputting a third signal and a fourth signal which is shifted from thethird signal by a phase of 90° and each of the third signal and thefourth signal having an amplitude corresponding to the controlled secondcurrent value; output means for outputting a first differential signalbetween the first and third signals and a second differential signalbetween the second and fourth signals; and a power supply for supplyinga preset voltage to said first and second current control means and saidfirst and second phase shifting means.
 11. An orthogonal signalgeneration system according to claim 10, wherein said first currentcontrol means includes a first linear element connected between saidpower supply and said first variable current source, said second currentcontrol means includes a second linear element connected between saidpower supply and said second variable current source, said first phaseshifting means includes a first integrator and a first differentiatorwhich are connected in parallel with said first linear element, and saidsecond phase shifting means includes a second integrator and a seconddifferentiator which are connected in parallel with said second linearelement.
 12. An orthogonal signal generation system according to claim11, wherein each of said first and second linear elements includes aninductor, each of said first integrator and first differentiator has acapacitor connected in parallel with said inductor of said first linearelement, each of said second integrator and second differentiator has acapacitor connected in parallel with said inductor of said second linearelement, and a resonance frequency of a parallel circuit of said firstlinear element, first integrator and first differentiator and aresonance frequency of a parallel circuit of said second linear element,second integrator and second differentiator are set equal to a frequencyof the input signal.
 13. An orthogonal signal generation systemaccording to claim 11, wherein each of said first and second linearelements includes a resistor.
 14. An orthogonal signal generation systemaccording to claim 10, wherein each of said first and second integratorsand said first and second differentiators includes a capacitor and aresistor.
 15. An orthogonal signal generation system according to claim10, wherein output means includes means for deriving a differencebetween the first and third signals and means for deriving a differencebetween the second and fourth signals.
 16. A communication device ofsuper heterodyne system comprising:demodulating means for demodulatingsignals, comprising,first current control means including a firstvariable current source and a first liner element connected to saidvariable current source, for controlling a first current value accordingto an input signal, second current control means including a secondvariable current source and a second liner element connected to saidvariable current source, for controlling a second current valueaccording to an inverted signal of the input signal, first phaseshifting means, including a first integrator and a first differentiatorwhich are connected in parallel with said first liner element, foroutputting a first signal and a second signal which is shifted from thefirst signal by a phase of 90° and each of the first signal and thesecond signal having an amplitude corresponding to the controlled firstcurrent value, second phase shifting means, including a secondintegrator and a second differentiator which are connected in parallelwith said second liner element, for outputting a third signal and afourth signal which is shifted from the third signal by a phase of 90°and each of the third signal and the fourth signal having an amplitudecorresponding to the controlled second current value, output means foroutputting a first differential signal between the first and thirdsignals and a second differential signal between the second and fourthsignals, and a power supply, connected to said first liner element andsaid second liner element, for supplying a preset voltage to said firstand second current control means and said first and second phaseshifting means.
 17. A communication device of direct conversion systemcomprising:frequency converting means for converting a base band signalof a transmission portion of said device into an RF signal and forconverting an RF signal of a reception portion of said device into abase band signal, comprising,first current control means including afirst variable current source and a first liner element connected tosaid variable current source, for controlling a first current valueaccording to an input signal, second current control means including asecond variable current source and a second liner element connected tosaid variable current source, for controlling a second current valueaccording to an inverted signal of the input signal, first phaseshifting means, including a first integrator and a first differentiatorwhich are connected in parallel with said first liner element, foroutputting a first signal and a second signal which is shifted from thefirst signal by a phase of 90° and each of the first signal and thesecond signal having an amplitude corresponding to the controlled firstcurrent value, second phase shifting means, including a secondintegrator and a second differentiator which are connected in parallelwith said second liner element, for outputting a third signal and afourth signal which is shifted from the third signal by a phase of 90°and each of the third signal and the fourth signal having an amplitudecorresponding to the controlled second current value, output means foroutputting a first differential signal between the first and thirdsignals and a second differential signal between the second and fourthsignals, and a power supply, connected to said first liner element andsaid second liner element, for supplying a preset voltage to said firstand second current control means and said first and second phaseshifting means.
 18. A demodulator comprising:first current control meansincluding a first variable current source and a first liner elementconnected to said variable current source, for inputting an RF (radiofrequency) signal as an input signal and controlling a first currentvalue according to the input signal; second current control meansincluding a second variable current source and a second liner elementconnected to said variable current source, for controlling a secondcurrent value according to an inverted signal of the input signal; firstphase shifting means, including a first integrator and a firstdifferentiator which are connected in parallel with said first linerelement, for outputting a first signal and a second signal which isshifted from the first signal by a phase of 90° and each of the firstsignal and the second signal having an amplitude corresponding to thecontrolled first current value; second phase shifting means, including asecond integrator and a second differentiator which are connected inparallel with said second liner element, for outputting a third signaland a fourth signal which is shifted from the third signal by a phase of90° and each of the third signal and the fourth signal having anamplitude corresponding to the controlled second current value; outputmeans for outputting a first differential signal between the first andthird signals and a second differential signal between the second andfourth signals; a power supply, connected to said first liner elementand said second liner element, for supplying a preset voltage to saidfirst and second current control means and said first and second phaseshifting means; and multiply means, for multiplying the input localsignal and the first differential signal and for multiplying the inputlocal signal and the second differential signal.